Encounter Pair Research Articles

The stability of unevenly spaced planetary systems

Studying the orbital stability of multi-planet systems is essential to understand planet formation, estimate the stable time of an observed planetary system, and advance population synthesis models. Although previous studies have primarily focused on ideal systems characterized by uniform orbital separations, in reality a diverse range of orbital separations exists among planets within the same system. This study focuses on investigating the dynamical stability of systems with non-uniform separation. We considered a system with 10 planets with masses of 10−7 solar masses around a central star with a mass of 1 solar mass. We performed more than 100,000 runs of N-body simulations with different parameters. Results demonstrate that reducing merely one pair of planetary spacing leads to an order of magnitude shorter orbital crossing times that could be formulated based on the Keplerian periods of the closest separation pair. Furthermore, the first collisions are found to be closely associated with the first encounter pair that is likely to be the closest separation pair initially. We conclude that when estimating the orbital crossing time and colliding pairs in a realistic situation, updating the formula derived for evenly spaced systems would be necessary.

Identifying the essential roles of light and sonication in dual‐stimuli regulated bulk atom transfer radical polymerization by multiscale simulation

AbstractMore and more attention has been paid to the important roles of external fields in controlled radical polymerization (CRP). However, their essential roles have not been studied thoroughly yet, which hinders the in‐depth understanding of the mechanism and kinetics. Herein, a strategy combining quantum chemical calculations (QCC) and kinetic modeling was adopted to identify the essential roles of light and ultrasonication in the dual‐stimuli regulated bulk atom transfer radical polymerization (ATRP). At the molecular level, the impact of light on Cu‐catalyzed ATRP was investigated. The CuIIBr/Me6TREN has high absorbance at an experimental wavelength of 365 nm (main excitation S0 → D7 accounts for 84.93%). Electron transfer reactions involving Me6TREN are more favorable paths for photochemical reactions, and the mechanism of the copper activation/deactivation pathway is inner sphere electron transfer. At the micro‐scale, a kinetic model based on the method of moment was established with a “series” encounter pair model to consider the influence of ultrasound on diffusional limitation. Simulation results show that the changes of the reaction rate coefficients ka, kda, and kt at high conversion reflect the degree of diffusional limitation by ultrasound. The multiscale modeling strategy applied in this study identifies the essential roles of photo and ultrasonication in dual‐stimuli regulated model systems, which can be extended to other external‐field regulated CRP to improve the mechanistic understanding.

Parametric Study of Structured UTM Separation Recommendations with Physics-Based Monte Carlo Distribution for Collision Risk Model

With the increasing demand for unmanned aircraft system (UAS) traffic management (UTM) airspace comes the need to ensure the safe operation and management of said airspace. One layer of defense against mid-air-collision and the ensuing third-party injury or fatality is the pre-flight separation assurance. This could be achieved by establishing the separation requirements for the UTM traffic based on the flight dynamics and communication navigation surveillance (CNS) performance that could be achieved in the airspace in question. A modified Reich collision risk model, typically used in civil aviation for separation minima evaluation, was used for the evaluation of the initial separation that would meet the target level of safety within a prescribed look-ahead time. This paper presents the parametric evaluation of using this physics-based and Monte Carlo-driven Reich collision risk model to evaluate the separation recommendation needed to achieve 10−7 mid-air-collision risk in UTM. The evaluation was conducted for an encounter pair consisting of identical ∼1.2 kg quadrotors with various encounter geometries, cruise velocities, navigation uncertainties, and communication latency.

Parallelization of the Symplectic Massive Body Algorithm (SyMBA) N-body Code

Direct N-body simulations of a large number of particles, especially in the study of planetesimal dynamics and planet formation, have been computationally challenging even with modern machines. This work presents the combination of fully parallelized N 2/2 interactions and the incorporation of the GENGA code’s close encounter pair grouping strategy to enable MIMD parallelization of the Symplectic Massive Body Algorithm (SyMBA) with OpenMP on multi-core CPUs in shared-memory environment. SyMBAp (SyMBA parallelized) preserves the symplectic nature of SyMBA and shows good scalability, with a speedup of 30.8 times with 56 cores in a simulation with 5000 fully interactive particles.

Ligands Mediate Anion Exchange between Colloidal Lead-Halide Perovskite Nanocrystals.

The soft lattice of lead-halide perovskite nanocrystals (NCs) allows tuning their optoelectronic characteristics via anion exchange by introducing halide salts to a solution of perovskite NCs. Similarly, cross-anion exchange can occur upon mixing NCs of different perovskite halides. This process, though, is detrimental for applications requiring perovskite NCs with different halides in close proximity. We study the effects of various stabilizing surface ligands on the kinetics of the cross-anion exchange reaction, comparing zwitterionic and ionic ligands. The kinetic analysis, inspired by the “cage effect” for solution reactions, showcases a mechanism where the surface capping ligands act as anion carriers that diffuse to the NC surface, forming an encounter pair enclosed by the surrounding ligands that initiates the anion exchange process. The zwitterionic ligands considerably slow down the cross-anion exchange process, and while they do not fully inhibit it, they confer improved stability alongside enhanced solubility relevant for various applications.

Software-in-the-loop investigation of wake-vortex-encounter-response of identical multirotor pair with PX4 attitude controller

Pairwise multirotor wake-encounter response studies with identical leader-follower unmanned aerial system (UAS) were conducted for a pair of Phantom 3 sized (∼1.2 kg) multirotor UAS with PX4 attitude controller used on the follower UAS. This is accomplished in the form of one-way coupled wake field from computational fluid dynamic (CFD) solution to Simulink software-in-the-loop (SITL) simulations. The study investigates resulting wake response as influenced by the substitution of idealized Lamb-Oseen vortex pair as stand-in for the CFD generated wake-vortex field produced by the leader UAS, as influenced by lateral positioning of the follower UAS relative to the track centerline, and as influenced by the follow distance between the encounter pair. The simulation results showed that the substitution of CFD with Lamb-Oseen vortex pair lacks flow along the vortex-axial direction, thus did not trigger the dynamic instability seen when the UAS was disturbed in all three rotational axis that was seen with CFD wake field. The dynamic instability of PX4 attitude controller in CFD wake field could be corrected by increasing the derivative gain for the yaw-rate. The investigations also showed that the maximum initial induced tilt angle, which is also the maximum value observed throughout, is highest when the follower UAS is positioned at a lateral offset equal to the distance between the wake-vortex core and the track centerline with the smallest follow distance feasible for the simulation to take place.

In silico mechanically mediated atom transfer radical polymerization: A detailed kinetic study

AbstractMechanically mediated atom transfer radical polymerization (mechanoATRP) utilizing ultrasound to generate activators and improve the diffusivity of macromolecular chains is introduced as an innovative externally controlled ATRP. Herein, a comprehensive kinetic model with free volume theory based “series” encounter pair model accounting for diffusional limitations on termination, activation, and deactivation is developed for the mechanoATRP of methyl acrylate. Comparative study by using different diffusion models, for example, wp model and reduced composite kt model, as well as constant apparent kjapp model confirms the goodness of the as‐developed model. Critically, mechanochemically induced reduction rate coefficient kr,s as a key kinetic parameter is associated with experimental conditions excluding the sonication effect by a fitting equation for the first time. In silico tracking of polymer dispersity with the help of kinetic model shows a better result compared with that by the classical dispersity equation. By defining an ultrasonic factor γj, a qualitative analysis for the effect of ultrasound conditions on the diffusional limitation in mechanoATRP is presented.

Joining Datasets Without Identifiers: Probabilistic Linkage of Virtual Pediatric Systems and PEDSnet.

To 1) probabilistically link two important pediatric data sources, Virtual Pediatric Systems and PEDSnet, 2) evaluate linkage accuracy overall and in patients with severe sepsis or septic shock, and 3) identify variables important to linkage accuracy. Retrospective linkage of prospectively collected datasets from Virtual Pediatrics Systems, Inc (Los Angeles, CA) and the PEDSnet consortium. Single-center academic PICU. All PICU encounters between January 1, 2012, and December 31, 2017, that were deterministically matched between the two datasets. None. We abstracted records from Virtual Pediatric Systems and PEDSnet corresponding to PICU encounters and probabilistically linked using 44 features shared by the two datasets. We generated a gold standard deterministic linkage using protected health information elements, which were then removed from datasets. We then calculated candidate pair log-likelihood ratios for all pairs of subjects and selected optimal pairs in a two-stage algorithm. A total of 22,051 gold standard PICU encounter pairs were identified over the study period. The optimal linkage model demonstrated excellent discrimination (area under the receiver operating characteristic curve > 0.99); 19,801 cases (89.9%) were matched with 13 false positives. The addition of two protected health information dates (admission month, birth day-of-year) increased to 20,189 (91.6%) the cases matched, with three false positives. Restricting to patients with Virtual Pediatric Systems diagnosis of severe sepsis or septic shock (n = 1,340 [6.1%]) matched 1,250 cases (93.2%) with zero false positives. Increased number of laboratory values present in the first 12 hours of admission significantly increased log-likelihood ratios, suggesting stronger candidate pair matching. We demonstrated the use of probabilistic linkage to accurately join two complementary pediatric critical care datasets at a single academic PICU in the absence of protected health information. Combining datasets with curated diagnoses and granular measurements can validate patient acuity metrics and facilitate multicenter machine learning algorithms. We anticipate these methods will generalize to other common PICU diagnoses.

Tungsten Iodide Clusters as Singlet Oxygen Photosensitizers: Exploring the Domain of Resonant Energy Transfer at 1 eV.

The photophysics of selected tungsten iodide clusters was examined with respect to their role as a photosensitizer for the production of singlet oxygen, O2(a1Δg). We examined all-iodo octahedral clusters, [W6I8(I6)]2-, and ligand-substituted octahedral clusters, [W6I8(L6)]2-, in which the ligand, L, occupies the outer apical positions surrounding the cluster core. We also examined a square-pyramidal cluster, [W5I8(I5)]-, in which the tungsten core was presumably more accessible to diffusional encounter with ground state oxygen, O2(X3Σg-). For the compounds examined, we find pronounced cluster-dependent changes in the yield of photosensitized O2(a1Δg) production. In particular, although the iodine-encased octahedral cluster, [W6I8(I6)]2-, is an efficient O2(a1Δg) sensitizer, the pyramidal cluster, [W5I8(I5)]-, does not make O2(a1Δg) at all. The latter provides fundamental insight into the important case where the sensitizer triplet state is nearly degenerate with the O2(X3Σg-)-O2(a1Δg) transition energy at 1 eV. Our data indicate that even with near resonance, energy transfer to form O2(a1Δg) will not occur within the 3sensitizer-O2(X3Σg-) encounter pair if other more efficient channels for energy dissipation are available.

Magnetic field effect on the oxidation of organic substances by molecular oxygen

AbstractMagnetic field effect (ME) on certain steps of chain oxidation of organic compounds of different series by molecular oxygen in liquid phase (initiation, chain propagation, and termination) is presented as a function of [O2] in two regimes. At low concentration of oxygen (<1.5 mM), magnetic field accelerates oxidation by 10% to 100%, but at high concentration no magnetic effect is detected. The dominating contribution into the magnetic effect (ME) is provided by reaction of alkyl radicals to oxygen: R• + O2 → RO2•. This reaction is stimulated by the Zeeman interaction which transforms in the encounter pair (R• O2) producing transformation of quartet spin state, which is spin forbidden to react, into the doublet spin state, which is spin allowed. At high concentration of oxygen, this reaction is not a limiting step and provides no magnetic effect on the total rate of oxidation.

Exchange interaction of O2 and nitroxide radical encounter pair as studied by time-resolved near IR luminescence spectroscopy of O2(1Δg)

Phosphorescence spectral shift of singlet oxygen, O2(1Δg) was studied in the presence of a nitroxide radical quencher. The O2-nitroxide complex causes a blue-shifts in both molecular solvents and in ionic liquids. The amount of shift is found to be nearly the same for both solvents, about 50 cm−1. This shift was attributed to quartet-doublet energy separation of O2(3Σg−)-nitroxide pair by exchange interaction of O2 and nitroxide radical.

Patient and doctor perspectives on HIV screening in the emergency department: A prospective cross-sectional study

BackgroundThe emergency department (ED) is mentioned specifically in the Swiss HIV testing recommendations as a site at which patients can benefit from expanded HIV testing to optimise early HIV diagnosis. At our centre, where local HIV seroprevalence is 0.2–0.4%, 1% of all patients presenting to the ED are tested for HIV. Barriers to HIV testing, from the patient and doctor perspective, and patient acceptability of rapid HIV testing were examined in this study.MethodsBetween October 2014 and May 2015, 100 discrete patient-doctor encounter pairs undertook a survey in the ED of Lausanne University Hospital, Switzerland. Patients completed a questionnaire on HIV risk factors and were offered free rapid HIV testing (INSTI™). For every patient included, the treating doctor was asked if HIV testing had 1) been indicated according to the national testing recommendations, 2) mentioned, and 3) offered during the consultation.ResultsOf 100 patients, 30 had indications for HIV testing through risk factors or a suggestive presenting complaint (PC). Fifty patients accepted rapid testing; no test was reactive. Of 50 patients declining testing, 82% considered themselves not at risk or had recently tested negative and 16% wished to focus on their PC. ED doctors identified 20 patients with testing indications, mentioned testing to nine and offered testing to six. The main reason for doctors not mentioning or not offering testing was the wish to focus on the PC.DiscussionPatients and doctors at our ED share the testing barrier of wishing to focus on the PC. Rapid HIV testing offered in parallel to the patient-doctor consultation increased the testing rate from 6% (offered by doctors) to 50%. Introducing this service would enable testing of patients not offered tests by their doctors and reduce missed opportunities for early HIV diagnosis.

Quantitative First-Principles Kinetic Modeling of the Aza-Michael Addition to Acrylates in Polar Aprotic Solvents.

This work presents a detailed computational study and kinetic analysis of the aza-Michael addition of primary and secondary amines to acrylates in an aprotic solvent. Accurate rate coefficients for all elementary steps in the various competing mechanisms are calculated using an ONIOM-based approach in which the full system is calculated with M06-2X/6-311+G(d,p) and the core system with CBS-QB3 corrected for solvation using COSMO-RS. Diffusional contributions are taken into account using the coupled encounter pair model with diffusion coefficients calculated based on molecular dynamics simulations. The calculated thermodynamic and kinetic parameters for all forward and reverse elementary reactions are fed to a microkinetic model giving excellent agreement with experimental data obtained using GC analysis. Rate analysis reveals that for primary and secondary amines, the aza-Michael addition to ethyl acrylate occurs preferentially according to a 1,2-addition mechanism, consisting of the pseudoequilibrated formation of a zwitterion followed by a rate controlling amine assisted proton transfer toward the singly substituted product. The alternative 1,4-addition becomes competitive if substituents are present on the amine or double bond of the acrylate. Primary amines react faster than secondary amines due to increased solvation of the zwitterionic intermediate and less sterically hindered proton transfer.

Computational Investigation of the Aminolysis of RAFT Macromolecules.

This work presents a detailed computational study and kinetic analysis of the aminolysis of dithioates, dithiobenzoates, trithiocarbonates, xanthates, and thiocarbamates, which are frequently used as chain-transfer agents for reversible addition-fragmentation chain-transfer (RAFT) polymerization. Rate coefficients were obtained from ab initio calculations, taking into account a diffusional contribution according to the encounter pair model. A kinetic model was constructed and reveals a reaction mechanism of four elementary steps: (i) formation of a zwitterionic intermediate, (ii) formation of a complex intermediate in which an assisting amine molecule takes over the proton from the zwitterionic intermediate, (iii) breakdown of the complex into a neutral tetrahedral intermediate with release of the assisting amine molecule, and (iv) amine-assisted breakdown of the neutral intermediate to the products. Furthermore, a comparative analysis indicates that the alkanedithioates and dithiobenzoates react the fastest, followed, respectively, by xanthates and trithiocarbonates, which react almost equally fast, and dithiocarbamates, which are not reactive at typical experimentally used conditions.

Coarse Master Equations for Binding Kinetics of Amyloid Peptide Dimers.

We characterize the kinetics of dimer formation of the short amyloid microcrystal-forming tetrapeptides NNQQ by constructing coarse master equations for the conformational dynamics of the system, using temperature replica-exchange molecular dynamics (REMD) simulations. We minimize the effects of Kramers-type recrossings by assigning conformational states based on their sequential time evolution. Transition rates are further estimated from short-time state propagators by maximizing the likelihood that the extracted rates agree with the observed atomistic trajectories without any a priori assumptions about their temperature dependence. Here, we evaluate the rates for both continuous replica trajectories that visit different temperatures and for discontinuous data corresponding to each REMD temperature. While the binding-unbinding kinetic process is clearly Markovian, the conformational dynamics of the bound NNQQ dimer has a complex character. Our kinetic analysis allows us to discriminate between short-lived encounter pairs and strongly bound conformational states. The conformational dynamics of NNQQ dimers supports a kinetically driven aggregation mechanism, in agreement with the polymorphic character reported for amyloid aggregates such as microcrystals and fibrils.

Carrier Dynamics in Pentacene|C60 Bilayer Solar Cell Investigated through the Magnetoconductance

We demonstrate the magnetoconductance (MC) effect originated from depressing the spin mixing in encounter pairs under the external magnetic field provides quantitative information about the singlet fission, the charge recombination, and the trap-related dynamics with triplet exciton in a bilayer device of pentacene (Pen) and fullerene (C60). Three MC effects in low-, moderate-, and high-fields were detected in the bilayer device at room temperature. Kinetic analysis of the low-field MC effect showed that the charge recombination yield at the Pen|C60 interface is ∼1%. Quantum mechanical simulations for dynamics of spin-carrying pairs following the conservation rule of spin angular momentum in recombination showed that the moderate- and high-field MC effects are caused by, respectively, the trap-related dynamics with triplet exciton and the singlet fission with a maximum yield of 52% in the layers. The quantitative information obtained by investigating the MC effect will contribute to the development of hig...

Structure learning and the Occam's razor principle: a new view of human function acquisition.

We often encounter pairs of variables in the world whose mutual relationship can be described by a function. After training, human responses closely correspond to these functional relationships. Here we study how humans predict unobserved segments of a function that they have been trained on and we compare how human predictions differ to those made by various function-learning models in the literature. Participants' performance was best predicted by the polynomial functions that generated the observations. Further, participants were able to explicitly report the correct generating function in most cases upon a post-experiment survey. This suggests that humans can abstract functions. To understand how they do so, we modeled human learning using an hierarchical Bayesian framework organized at two levels of abstraction: function learning and parameter learning, and used it to understand the time course of participants' learning as we surreptitiously changed the generating function over time. This Bayesian model selection framework allowed us to analyze the time course of function learning and parameter learning in relative isolation. We found that participants acquired new functions as they changed and even when parameter learning was not completely accurate, the probability that the correct function was learned remained high. Most importantly, we found that humans selected the simplest-fitting function with the highest probability and that they acquired simpler functions faster than more complex ones. Both aspects of this behavior, extent and rate of selection, present evidence that human function learning obeys the Occam's razor principle.

Photophysics and halide quenching of a cationic metalloporphyrin in water

In this work, the steady state S0-S1 and S0-S2 absorption and emission behaviour of the water-soluble tetrakis(N-methyl-4-pyridyl)porphyrin zinc(ii) tetrachloride (ZnTMPyP) in media of constant and high ionic strength, both with and without iodide ions as a fluorescence quencher, was measured. The quenching of the ZnTMPyP S1 state by iodide ions proceeds primarily through diffusion-limited interaction in an encounter pair but the formation of a loose association between the ZnTMPyP S1 state and iodide ions also provides a minor quenching pathway. The ZnTMPyP S2 state was quenched minimally by iodide, likely through an electron transfer mechanism at an average donor-acceptor distance of ~0.7 nm. The results presented here highlight the notion that significant iodide quenching of the ZnTMPyP S1 state can be a source of inefficiencies in porphyrin-based dye-sensitized solar cells. The minimal quenching of the S2 state indicates that no significant loss of efficiency will be introduced by using iodide as component of the electrolyte system in upconversion by triplet-triplet annihilation (UC-TTA) enhanced solar cells.

The Crucial Role of Diffusional Limitations in Controlled Radical Polymerization

AbstractThe importance of diffusional limitations in controlled radical polymerization at laboratory scale and in homogeneous medium is highlighted using different diffusion models. In particular, a coupled ‘parallel’ encounter pair model is introduced which allows a rigorous description of the influence of diffusional limitations on the activation/deactivation process. Diffusional limitations on termination have a significant influence, whereas those on the activation/deactivation process can disturb the regular growth pattern at high conversion if the mediating agent is a sufficiently bulky species. Diffusional limitations on initiation are only important in case radical initiator is still present at high conversion.

Theoretical Investigation into the Mechanism of 3′-dGMP Oxidation by [PtIVCl4(dach)

The mechanism for the oxidation of 3'-dGMP by [PtCl(4)(dach)] (dach = diaminocyclohexane) in the presence of [PtCl(2)(dach)] has been investigated using density functional theory. We find that the initial complexation, i.e., the formation of [PtCl(3)(dach)(3'-dGMP)], is greatly assisted by the reaction of the encounter pair [PtCl(2)(dach)···3'-dGMP] with [PtCl(4)(dach)], leading to migration of an axial chlorine ligand from platinum(IV) to platinum(II). A dinuclear platinum(II)/platinum(IV) intermediate could not be found, but the reaction is predicted to pass through a platinum(III)/platinum(III) transition structure. A cyclization process, i.e., C8-O bond formation, from [PtCl(3)(dach)(3'-dGMP)] occurs through an intriguing phosphate-water-assisted deprotonation reaction, analogous to the opposite of a proton shuttle mechanism. Followed by this, the guanine moiety is oxidized via dissociation of the Pt(IV)-Cl(ax) bond, and the cyclic ether product is finally formed after deprotonation. We have provided rationalizations, including molecular orbital explanations, for the key steps in the process. Our results help to explain the effect of [PtCl(4)(dach)] on the complexation step and the effect of a strong hydroxide base on the cyclization reaction. The overall reaction cycle is intricate and involves autocatalysis by a platinum(II) species.